Annunciator systems have been developed for monitoring and displaying operating conditions in complex plant systems. The typical annunciator system might include a plurality of gauges for indicating variable parameters such as pressures, temperatures, quantities, or the like for illustrating the various conditions in the process plant. When a complex process is displayed by an annunciator system, the various details and indicators may be so many in number and so complex in their arrangement that a graphic display is desirable in order for the plant operators to understand and easily read the various indicators. A graphic display of this kind usually includes a translucent panel positioned in or near the annunciator cabinet and displaying symbols representing pumps, valves, parameters such as liquid levels or fluid pressures, other plant equipment and conditions, and flow lines extending between the various symbols. The various symbols on the display panel are illuminated by lamps or other light sources placed behind the display panel, and the light emitted by the lamps represent various plant conditions. For example, when a lamp illuminates the symbol of a pump on the display panel, it would indicate a certain condition existing in the pump; e.g., if the pump were in operation, overheated, or not operating properly. Similarly, one or several lamps might illuminate the symbol of a storage tank to indicate an empty, properly filled, or overfilled condition in the storage tank. Associated with such annunciator systems are control systems by which the process plant can be controlled in response to the conditions indicated by the annunciator display and monitoring operations. Examples of typical graphic display annunciator systems and related control devices of the prior art are shown in U.S. Pat. Nos. 3,624,648 to Willoughby and 3,754,245 to Peprnick.
The lamps placed behind the display panel often are mounted onto a backing panel spaced behind the display panel in locations aligned with the graphic symbols representing the process or condition selectively illuminated by each such lamp. Each lamp generally is carried by a socket for mounting on the backing panel. In such annunciator systems, the ability to modify the process plant display or flow chart is limited and costly. Each time the operational process or flow represented by the display panel is modified, e.g., a pump moved or valve sequence rearranged, a new graphic display must be laid out and the lamps usually must be repositioned on the backing panel to coincide with different locations on the graphic display. Whenever a symbol of representation on the display panel is relocated, a corresponding new position for the lamp must be identified on the backing panel so as to juxtapose each lamp with the corresponding graphic symbol on the display panel. The graphic display panel unit must at least be partially opened or taken apart to separate the backing panel from the display panel in order to relocate the lamp on the backing panel. Determining the proper mounting location on the backing panel is, in practice, often a guessing game as the person revising the graphic display panel must work within the partially disassembled unit. That person relocates the lamp on the backing panel to a position which appears at least approximately aligned with the corresponding graphic symbol on the display panel, and then assembles (or reassembles) the display panel and tests the lamp to see whether lamp and graphic symbols are indeed aligned. Typically, the lamp will be somewhat misaligned with the corresponding graphic symbol, and the person must again take the panel apart to adjust the location of the lamp on the backing panel. This process may require several iterations for each lamp in a display panel typically containing numerous lamps requiring alignment, either when the display panel is initially built or subsequently modified. Consequently, there is a need for a graphic display panel which provides localized internal lighting that is easily repositionable.